Optical recording medium and optical recording medium evaluation method

ABSTRACT

A conventional mechanical precision measuring device is used effectively in the measurement of the eccentricity of a high density optical disc. A DVR optical disc  2  comprises a pattern  4  of fine depressions and protrusions conformed to the intended format of the disc (a first depressions and protrusions: pits and/or grooves) formed within an information recording area  6  of the disc, as well as a pattern  8  of fine depressions and protrusions for measuring the eccentricity (a second depressions and protrusions), which is formed in an area other than the information recording area  6 , and is formed in accordance with a CD format having lower recording density than that of the format defined for the DVR optical disc  2 . By using this pattern  8  of fine depressions and protrusions for mechanical detection, conventional mechanical precision measuring devices designed for the CD family can be used as it is, for measuring the eccentricity (mechanical precision) of the DVR optical disc  2.

TECHNICAL FIELD

[0001] The present invention relates to an optical recording medium, andmore particularly to an optical recording medium capable of recordingand playback using an optical system that uses a blue laser, and amethod of evaluating the mechanical precision thereof.

[0002] In this description, the term optical recording medium (opticaldisc) includes not only completed optical recording media (opticaldiscs), but also semi-completed disc substrates.

BACKGROUND ART

[0003] Optical discs such as CD, MD, and DVD comprise an informationrecording area formed from a pattern of fine depressions and protrusions(pits and/or grooves), which are formed in accordance with apredetermined format defined for each disc.

[0004] In order to measure the degree of eccentricity relative to thecenter of rotation for this optical disc pattern of fine depressions andprotrusions, conventionally a mechanical precision measuring device suchas an eccentricity measuring device or an axial displacement measuringdevice has been used.

[0005] A variety of mechanical precision measuring devices have beenproposed.

[0006] It is a known representative structure in which a laser beam isirradiated through an objective lens that can be driven by an actuator,on the fine depressions and protrusions of the optical disc. In thestructure, the laser beam tracks the depressions and protrusions, sothat the movement (the degree of displacement) of the objective lens,which reflects the displacement in a radial direction (the eccentricity)of the fine depressions and protrusions, is detected as a fluctuation inthe beam spot position on an optical position sensor (for example, seeJapanese Patent Laid-Open Publication No. 1987-117151).

[0007] In recent years, optical discs capable of recording and playbackusing optical systems that use blue lasers have been proposed, includingthe DVR system which uses an optical system with laser light ofwavelength 405 nm, and a NA value of 0.85.

[0008] However, conventional mechanical precision measuring devices usedfor CD or DVD discs have been produced on the basis of CD or DVD formatproperties (for example, a laser light wavelength λ of 780 nm, anumerical aperture NA=0.45, and a groove pitch of 1.6 μm in the case ofCD, and 650 nm, NA=0.6, and a groove pitch of 0.74 nm in the case ofDVD). Therefore, if the groove pitch of the pattern of fine depressionsand protrusions is reduced to 500 nm or less for example, then thepattern becomes difficult to track so that these conventional devicescannot be adopted as measuring devices for DVR.

[0009] Of course, the eccentricity of these types of high densityoptical discs can be measured by using a DVR (higher performance)mechanical precision measuring device having similar structure to thatfor CD or DVD.

[0010] However, these types of mechanical precision measuring devicesare generally expensive, even for CD media. Products for DVR media areeven more expensive. If a new device is required for each new format,then large costs are unavoidable. Moreover, another problem arises inthat if a new measuring device is purchased, the existing mechanicalprecision measuring devices used for measuring CD or DVD serve as asurplus.

[0011] On the other hand, the measurement and evaluation of optical disceccentricity by inspection under a microscope, not by measurement with amechanical precision measuring device, has also been frequentlyperformed. However, even in these microscope measurement methods, whenthe groove pitch of the pattern of fine depressions and protrusions isreduced to 500 nm or less, the required light diffraction phenomenon ishard to be obtained so that the measurement itself becomes moredifficult.

DISCLOSURE OF THE INVENTION

[0012] The present invention aims to resolve these conventionalproblems, with an object of providing an optical disc or an eccentricitymeasuring system, which enables conventional mechanical precisionmeasuring devices to be effectively used as it is, in the measurement ofthe mechanical precision such as the eccentricity of an optical discconformed to a high density format, and also enables conventionaleccentricity inspection and the like using a microscope.

[0013] The present invention is able to achieve the above object byproposing an optical disc according to a construction (1) describedbelow.

[0014] (1) An optical recording medium having grooves and/or pits as afirst depressions and protrusions, of which width is equal to or lessthan 200 nm, in a main information recording area, the optical recordingmedium comprising grooves and/or lands as a second depressions andprotrusions in a predetermined position other than the main informationrecording area, the grooves and/or lands as the second depressions andprotrusions being able to be tracked by an optical system in which alaser wavelength thereof is 780±10 nm, and a numerical aperture NA ofthe recording and playback lens of is 0.45±0.01.

[0015] The groove widths listed in this description all refer to halfwidths.

[0016] With the present invention, it is noticed that as long as twoformats described above are either formed consecutively or formed withina single process (the same disc setting), the precision required inrelation to the optical disc eccentricity (the mechanical precision)does not change significantly, even if the optical disc format ischanged.

[0017] Based on this finding, rather than simply purchasing new highperformance mechanical precision measuring devices designed to measurehigher density optical discs, the present invention employs the reversethinking, and adjusts the specifications of the optical disc beingprepared so as to be applied to mechanical precision measuring deviceswhich are conventionally used.

[0018] An optical disc according to the present invention comprises apattern of fine depressions and protrusions (pits and/or grooves)conformed to the intended format of the optical disc, formed within aninformation recording area of the disc, as well as a pattern of finedepressions and protrusions for mechanical detection, which is formed inan area other than the information recording area, and is formed inaccordance with a format with a lower recording density (a wider groovewidth and a larger groove pitch) than that of the defined format of theoptical disc.

[0019] As a result, when a high density optical disc such as a so-calledDVR disc is produced, by using this pattern of fine depressions andprotrusions for mechanical detection, conventional mechanical precisionmeasuring devices developed for the CD or DVD families can be usedwithout modification.

[0020] In other words, the eccentricity of a wide variety of opticaldiscs with different track shapes (including properties such as thepitch, width or depth of the pits and/or grooves) can be measured usinga single mechanical precision measuring device.

[0021] The pattern of fine depressions and protrusions formed in an areaother than the information recording area can be inspected bymicroscope, by making the specification into CD level, since the pitchcan be set at a level which enables to recognize a clear lightdiffraction phenomenon. Accordingly, even in those situations in whicheven a mechanical precision measuring device developed for the CD familyof discs cannot be purchased, if only a microscope is available,eccentricity can still be measured in a conventional manner.

[0022] The following types of structures could be envisaged asvariations of the present invention. Details of these variations aregiven below.

[0023] (2) The optical recording medium according to claim 1, wherein agroove pitch of the first depressions and protrusions is equal to orless than 500 nm.

[0024] (3) The optical recording medium according to claim 1 or 2,wherein a groove width of the second depressions and protrusions iswithin a range from 400 nm to 600 nm, and a groove pitch is within arange from 1.2 μm to 2.0 μm.

[0025] (4) The optical recording medium according to any one of claims 1to 3, wherein the first depressions and protrusions can be recorded to,and/or played back from using an optical system in which a laserwavelength thereof is 450 nm or less, and a numerical aperture NA forthe recording and playback lens is 0.7 or more.

[0026] (5) An optical recording medium having grooves and/or pits as afirst depressions and protrusions, of which width is equal to or lessthan 200 nm, in a main information recording area, the optical recordingmedium comprising grooves and/or lands as a second depressions andprotrusions in a predetermined position other than the main informationrecording area, the grooves and/or lands as the second depressions andprotrusions being able to be tracked by an optical system in which alaser wavelength thereof is 640±20 nm, and a numerical aperture NA ofthe recording and playback lens is 0.6±0.01.

[0027] (6) 6. The optical recording medium according to claim 5, whereina groove pitch of the first section of depressions and protrusions isequal to or less than 500 nm.

[0028] (7) The optical recording medium according to claim 5 or 6,wherein a groove width of the second section of depressions andprotrusions is within a range from 250 nm to 750 nm, and a groove pitchis within a range from 0.6 μm to 1.5 μm.

[0029] (8) The optical recording medium according to any one of claims 5to 7, wherein the first section of depressions and protrusions can berecorded to, and/or played back from using an optical system in which alaser wavelength thereof is 450 nm or less, and a numerical aperture NAfor the recording and playback lens is 0.7 or more.

[0030] (9) A method of evaluating an optical recording medium, forevaluating mechanical precision of grooves and/or pits of a firstsection of depressions and protrusions, which functions as a maininformation recording area and can be recorded to, and/or played backfrom using an optical system with a laser wavelength of 450 nm or less,and a numerical aperture NA for the recording and playback lens of 0.7or more, wherein a second section of depressions and protrusions, whichis provided in a predetermined position outside the informationrecording area, and can be tracked by any one of an optical system witha laser wavelength of 780±10 nm, and a numerical aperture NA of therecording and playback lens of 0.45±0.01, and an optical system with alaser wavelength of 640±20 nm, and a numerical aperture NA of therecording and playback lens of 0.6±0.01, is formed either concurrentlyor consecutively with the first section of depressions and protrusions,and the mechanical precision of the first section of depressions andprotrusions is evaluated by examining the second section of depressionsand protrusions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a cross-sectional view showing a schematic illustrationof a DVR optical disc used with an optical disc mechanical precisionmeasuring system of the present invention; and

[0032]FIG. 2 is a schematic structural diagram of a mechanical precisionmeasuring device used in the above measuring system.

BEST MODE FOR CARRYING OUT THE INVENTION

[0033] Hereinafter, an embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings.

[0034] The optical disc eccentricity measuring system according to thisembodiment applies the present invention to enable the eccentricity(mechanical precision) of a DVR optical disc to be measured using amechanical precision measuring device developed for CD discs.

[0035]FIG. 1 shows a schematic illustration of a DVR optical discsubstrate used with the present embodiment.

[0036] This DVR optical disc 2 comprises an information recording area 6in which a pattern 4 of fine depressions and protrusions (a firstdepressions and protrusions) is formed. In the case of this embodiment,this pattern 4 of fine depressions and protrusions within theinformation recording area 6 is formed only from grooves, with a depthof 20 to 50 nm, a width of 100 to 200 nm, and a radial pitch (groovepitch) of 200 to 400 nm. These values are conformed to the newlyproposed discs known as DVR.

[0037] On the other hand, in this embodiment, grooves comprising apattern 8 of fine depressions and protrusions used for measuring theeccentricity (a second depressions and protrusions) are formed spirallywithin an inner peripheral area 7A of the area 7 other than theinformation recording area 6. This pattern 8 of fine depressions andprotrusions used for measuring the eccentricity is formed during thesame process used for forming the pattern 4 of fine depressions andprotrusions of the information recording area 6, that is, during asingle disc setting. Because the depth of the grooves is dependent onthe thickness of the photoresist, the pattern 8 has a similar depth tothat of the grooves described above, whereas the width is from 400 to1000 nm, and the radial pitch (groove pitch) is from 1.2 to 2.0 μm.These values are conformed to the CD format.

[0038] The pattern 8 need not necessarily be formed in a spiral shape,and may also be formed concentrically, so that the pattern of finedepressions and protrusions used for measuring the eccentricity may beformed concentrically with the information recording area 6. Regardlessof the configuration adopted, the level of error settles within a rangethat can be effectively ignored. Furthermore, these pattern 8 may alsobe formed in an outer peripheral area 7B instead of the inner peripheralarea 7A.

[0039]FIG. 2 shows a schematic illustration of the structure of a CDmechanical precision measuring device 10 used in the eccentricitymeasuring system according to this embodiment.

[0040] This CD mechanical precision measuring device 10 is alreadyknown. In other words, it is one of the important characteristics of thepresent embodiment wherein a conventionally known CD mechanicalprecision measuring device 10 can be used as it is, for measuring theeccentricity of the DVR optical disc 2. The CD mechanical precisionmeasuring device 10 is the type of device that is already widely used inthe industry, and because the mechanical precision measuring device tobe used in the present invention is not restricted to a device of thisconstruction, the description below focuses mainly on the functions ofthe device.

[0041] This CD mechanical precision measuring device 10 comprises alaser beam source 12 for generating a laser beam with a wavelength of780 nm, in accordance with the CD format. The laser beam irradiated fromthis laser beam source 12 is split into two directions by a spectroscope14.

[0042] One of the split laser beams passes through an objective lens 16,and irradiates the pattern 8 of fine depressions and protrusions usedfor measuring the eccentricity, formed on the inner peripheral side 7Aof the information recording area 6 of the DVR optical disc 2. Thereflected light from the pattern 8 of fine depressions and protrusionsused for measuring the eccentricity is detected by a photodetector 18,and a focus and tracking control circuit 20 then drives an actuator 22based on this detection information. By driving this actuator 22, theobjective lens 16 is moved so that the laser beam tracks the pattern 8of fine depressions and protrusions used for measuring the eccentricity.

[0043] The laser beam is irradiated not onto the pattern 4 of finedepressions and protrusions within the actual information recording area6, but onto the pattern 8 of fine depressions and protrusions used formeasuring the eccentricity, which is formed on the inner peripheral side7A of the information recording area 6 in accordance with CD formatproperties. Therefore, no problems arise in tracking the pattern withthe performance provided by the CD mechanical precision measuring device10.

[0044] On the other hand, the other split laser beam is irradiated ontoa mirror 26 via a fixed lens 24. This mirror 26 is integrated with theobjective lens 16, and follows the radial displacement of the pattern 8of fine depressions and protrusions used for measuring the eccentricity.The reflected beam from the mirror 26 passes through a beam splitter 28and reaches an optical position sensor 30.

[0045] As the direction of the reflected beam from the mirror 26changes, the beam spot position on this optical position sensor 30 alsochanges. The direction of the reflected beam from the mirror 26 islinked with the movement of the objective lens 16, that is, the radialdisplacement of the pattern 8 of fine depressions and protrusions usedfor measuring the eccentricity. Namely, the direction of the reflectedbeam corresponds to the state of eccentricity for the DVR optical disc2. Accordingly, by detecting fluctuations in the beam spot position onthe optical position sensor 30, the state of eccentricity for the DVRoptical disc 2 can be determined.

[0046] The output from the optical position sensor 30 is amplified by anamplifier 32, and output as “eccentricity information”. As a result, thestate of eccentricity of the DVR optical disc 2 can be measured usingthe CD mechanical precision measuring device 10.

EXAMPLE 1

[0047] A stamper was prepared. The stamper comprised grooves with adepth of 30 nm, a width of 160 nm, and a radial groove pitch of 0.3 μm,in accordance with the properties of the newly proposed discs known asDVR, which was formed as the pattern of fine depressions and protrusionsof the information recording area (the first depressions andprotrusions). The stamper also comprised grooves with a depth of 30 nm,a width of 500 nm, and a radial groove pitch of 1.6 μm, in accordancewith the properties required for the CD format, on the inner peripheralside of the information recording area for measuring the eccentricity(the second depressions and protrusions).

[0048] A two beam cutting machine was used to form the first depressionsand protrusions from the inner periphery to the outer periphery, andthen, the cutting machine was returned to the innermost position to formthe second depressions and protrusions.

[0049] The stamper was mounted in a die assembly of an injection moldingmachine in order to mold a resin substrate with an outer diameter of 120mm, and a thickness of 1.2 mm (which belongs to the broad classificationof optical discs according to the present invention).

[0050] The resin used was polycarbonate H4000-N282, manufactured byMitsubishi Engineering-Plastics Corporation. The main molding conditionsincluded a resin melt temperature of 360° C., a mold temperature of 125°C., and a mold clamping force of 35 tons.

[0051] When this resin substrate was measured by using a CD mechanicalprecision measuring device described above (LM1200, manufactured by OnoSokki Co., Ltd., wavelength of laser beam 780 nm, numerical aperture NA0.45), the groove surface formed in the information recording area couldbe focused on, but could not be tracked on the radial direction,resulting measurement of the eccentricity was impossible.

[0052] In contrast, as to the grooves formed in accordance with the CDformat on the inner peripheral side of the resin substrate other thanthe information recording area, both focus-on and radial tracking(track-on) were possible, so that eccentricity could be measured with noproblems.

[0053] From these results, it is evident that by forming a pattern offine depressions and protrusions conformed to the CD format on an areaof a DVR optical disc other than the information recording area, theeccentricity of the DVR optical disc can be measured by using amechanical precision measuring device developed for CD.

EXAMPLE 2

[0054] Using the resin substrate of the example 1, the eccentricitythereof was measured by microscope inspection. As a result, the boundarylines between the information recording area (the first depressions andprotrusions) and the mirrored area were difficult to be detected by themicroscope, so that measurement of the eccentricity was impossible.However, the boundary lines between the pattern of fine depressions andprotrusions for measuring the eccentricity (the second depressions andprotrusions), formed other than the information recording area and themirrored area were able to be detected even by microscope, so that theeccentricity could be measured easily.

[0055] From these results, it is evident that regardless of the formatof the pattern of fine depressions and protrusions formed within theinformation recording area, as long as a pattern of fine depressions andprotrusions for measuring the eccentricity having a pitch of the CDformat level is formed on an area other than the information recordingarea, measurement of the eccentricity is also possible using amicroscope in a conventional manner.

[0056] Hence it is clear that even for an optical disc in the DVR level(an optical recording medium in which the width of grooves and/or pitsin a first depressions and protrusions which functions as the maininformation recording area is equal to or less than 200 nm), if either asecond depressions and protrusions of the CD level (a level in which thewidth of the grooves is at least 400 nm but no more than 1000 nm, andthe groove pitch is at least 1.2 μm but no more than 2.0 μm), or asecond depressions and protrusions of the DVD level (a level in whichthe width of the grooves is at least 250 nm but no more than 750 nm, andthe groove pitch is at least 0.6 μm but no more than 1.5 μm), isprovided in a position either on the inner periphery or the outerperiphery of the disc (a predetermined position other than theinformation recording area), then, the mechanical precision of the disccan be satisfactorily measured and evaluated by either a mechanicalprecision measuring device designed for CD or a mechanical precisionmeasuring device designed for DVD.

[0057] In other words, it is clear that even though the optical discitself is surely of the DVR level, with a groove pitch of 500 nm orless, and has properties which mean that recording and/or playback canonly be conducted with an optical system with a laser wavelength of 450nm or less, and a numerical aperture NA for the recording and playbacklens of 0.7 or more, the mechanical precision of the disc can still besatisfactorily measured and evaluated using either a mechanicalprecision measuring device designed for CD or a mechanical precisionmeasuring device designed for DVD.

[0058] The CD mechanical precision measuring device 10 described in theabove embodiment merely represents one possible example, and in thepresent invention, there are no particular restrictions on theconstruction of the mechanical precision measuring device. Specifically,any CD or DVD mechanical precision measuring device that has beenproposed, or is commercially available, can be used, and if new CD orDVD mechanical precision measuring devices, such as low cost devices,are developed in the future, these would also be applicable to thepresent invention.

INDUSTRIAL APPLICABILITY

[0059] According to the present invention, a conventional mechanicalprecision measuring device can be effectively used, as it is, in themeasurement of the mechanical precision of an optical disc with a highdensity format.

1. An optical recording medium having grooves and/or pits as a firstdepressions and protrusions, of which width is equal to or less than 200nm, in a main information recording area, the optical recording mediumcomprising grooves and/or lands as a second depressions and protrusionsin a predetermined position other than the main information recordingarea, the grooves and/or lands as the second depressions and protrusionsbeing able to be tracked by an optical system in which a laserwavelength thereof is 780±10 nm, and a numerical aperture NA of therecording and playback lens of is 0.45±0.01.
 2. The optical recordingmedium according to claim 1, wherein a groove pitch of the firstdepressions and protrusions is equal to or less than 500 nm.
 3. Theoptical recording medium according to claim 1, wherein a groove width ofthe second depressions and protrusions is within a range from 400 nm to600 nm, and a groove pitch is within a range from 1.2 μm to 2.0 μm. 4.The optical recording medium according to claim 1, wherein the firstdepressions and protrusions can be recorded to, and/or played back fromusing an optical system in which a laser wavelength thereof is 450 nm orless, and a numerical aperture NA for the recording and playback lens is0.7 or more.
 5. An optical recording medium having grooves and/or pitsas a first depressions and protrusions, of which width is equal to orless than 200 nm, in a main information recording area, the opticalrecording medium comprising grooves and/or lands as a second depressionsand protrusions in a predetermined position other than the maininformation recording area, the grooves and/or lands as the seconddepressions and protrusions being able to be tracked by an opticalsystem in which a laser wavelength thereof is 640±20 nm, and a numericalaperture NA of the recording and playback lens is 0.6±0.01.
 6. Theoptical recording medium according to claim 5, wherein a groove pitch ofthe first section of depressions and protrusions is equal to or lessthan 500 nm.
 7. The optical recording medium according to claim 5,wherein a groove width of the second section of depressions andprotrusions is within a range from 250 nm to 750 nm, and a groove pitchis within a range from 0.6 μm to 1.5 μm.
 8. The optical recording mediumaccording to claim 5, wherein the first section of depressions andprotrusions can be recorded to, and/or played back from using an opticalsystem in which a laser wavelength thereof is 450 nm or less, and anumerical aperture NA for the recording and playback lens is 0.7 ormore.
 9. A method of evaluating an optical recording medium, forevaluating mechanical precision of grooves and/or pits as a firstdepressions and protrusions, which functions as a main informationrecording area and can be recorded to, and/or played back from using anoptical system in which a laser wavelength thereof is 450 nm or less,and a numerical aperture NA for the recording and playback lens is 0.7or more, wherein, a second depressions and protrusions, which isprovided in a predetermined position other than the main informationrecording area, and can be tracked by any one of an optical system inwhich a laser wavelength thereof is 780±10 nm, and a numerical apertureNA of the recording and playback lens is 0.45±0.01, and an opticalsystem in which a laser wavelength thereof is 640±20 nm, and a numericalaperture NA of the recording and playback lens of 0.6±0.01, is formedeither concurrently or consecutively with the first depressions andprotrusions, and the mechanical precision of the first depressions andprotrusions is evaluated by examining the second depressions andprotrusions.
 10. The optical recording medium according to claim 2,wherein a groove width of the second depressions and protrusions iswithin a range from 400 nm to 600 nm, and a groove pitch is within arange from 1.2 μm to 2.0 μm.
 11. The optical recording medium accordingto claim 2, wherein the first depressions and protrusions can berecorded to, and/or played back from using an optical system in which alaser wavelength thereof is 450 nm or less, and a numerical aperture NAfor the recording and playback lens is 0.7 or more.
 12. The opticalrecording medium according to claim 3, wherein the first depressions andprotrusions can be recorded to, and/or played back from using an opticalsystem in which a laser wavelength thereof is 450 nm or less, and anumerical aperture NA for the recording and playback lens is 0.7 ormore.
 13. The optical recording medium according to claim 6, wherein agroove width of the second section of depressions and protrusions iswithin a range from 250 nm to 750 nm, and a groove pitch is within arange from 0.6 μm to 1.5 μm.
 14. The optical recording medium accordingto claim 6, wherein the first section of depressions and protrusions canbe recorded to, and/or played back from using an optical system in whicha laser wavelength thereof is 450 nm or less, and a numerical apertureNA for the recording and playback lens is 0.7 or more.
 15. The opticalrecording medium according to claim 7, wherein the first section ofdepressions and protrusions can be recorded to, and/or played back fromusing an optical system in which a laser wavelength thereof is 450 nm orless, and a numerical aperture NA for the recording and playback lens is0.7 or more.